Conventional belt conveyors typically have a plurality of scraper assemblies disposed to remove deposits adhering to the surface of the conveyor belt. For example, it is common to provide a scraper assembly at or near the head pulley to clean the belt as the belt begins its return travel. Additionally, trailing arm scrapers are commonly located along the length of the belt in between the head and tail pulleys to provide additional cleaning. U.S. Pat. No. 5,016,746, assigned to ASGCO Manufacturing, assignee of the present invention, illustrates scrapers of this type.
A common type of scraper blade assembly comprises a plurality of mounted scraper blades extending across the transverse axis of the conveyor belt. The blades are mounted on support arms which are fixed to and extend from a transverse rotatable support shaft. Rotation of the transverse support shaft moves the scraper blades into and out of contact with the conveyor belt. Applying a torque to the support shaft increases the contacting force of the scraper blades against the belt.
During installation and use of the prior art scraper blade assemblies, the support shaft is torqued and locked in position when a predetermined contacting force is exerted on the belt by the scraper blades. As the scraper blades wear down, the contacting force on the belt steadily decreases, thereby reducing the cleaning efficiency of the scraper blade assembly. Over the useful life of the scraper blades, the support shaft must be adjusted several times to compensate for wear on the blades. After prolonged use, standard clamps and set screws become ineffective to maintain and adjust the contacting force.
To afford facile adjustment of the scraper blades, Gordon, U.S. Pat. No. 4,533,036 teaches a pair of adjustment collars which control the torque exerted on the shaft by a torsion tube. One collar is fixed to a torsion tube; the other collar is fixed to a transverse support shaft. Each collar is provided with a series of radial apertures which register with each other as the collars are rotated. The ends of a U-shaped link are inserted in the pair of registered holes to lock the collars relative to each other and lock the support shaft in that position. When it is desired to adjust the contacting force, the link is removed, the collars are rotated to increase the force, and then the collars are locked when the torque exerted on the support shaft by the torsion tube causes the desired contacting/cleaning force on the belt.
Optimum cleaning efficiency of the scraper assembly is highly dependent upon the magnitude of the force exerted by the scraper blade against the belt. If the contacting force is too high, the blade will wear prematurely; if the contacting force is too low, the blade will not efficiently clean the belt. Thus, the capability of the tensioning device to lock the support shaft as close as possible to its optimum cleaning position greatly affects the efficiency of the scraper blade assembly.
Using the adjustment mechanism provided by Gordon, the torsion tube is adjusted to a position which provides the optimum contacting force on the belt. However, very often in this position a pair of holes on the collars are not aligned. To properly lock the support shaft, the operator must either over-torque or under-torque the torsion tube away from the optimum cleaning position until a pair of holes on the collars are in alignment. The resultant contacting force on the belt is then either greater or less than the optimum cleaning force.
The sensitivity of the tensioning device such as that taught in Gordon depends on the minimum spacing of the radial apertures. The sensitivity of the mechanism is increased by more closely spacing the apertures on the collars; however, practical design considerations such as the collar diameter and the diameter of the apertures limit the spacing between holes around the collar, and thus, the sensitivity of the adjustment. Typical mechanical adjustment units taught in the prior art are only adjustable in increments of approximately 11.25 degrees of rotation. Often the optimum contacting/cleaning force is attained when the shaft is rotated to a position somewhere in between the 11.25 degree settings.
It is therefore an object of the present invention to provide a tensioning device which is adjustable in increments significantly less than the minimum spacing between apertures on prior art tensioners, i.e., about 11.25 degrees, preferably about 40% of the 11.25 degree spacing, and better yet about 13% of the 11.25 degree spacing.